YIG (yttrium iron garnet) tuned filters are used in spectrum analyzers. The YIG tuned filter comprises at least one YIG ellipsoid, generally a sphere, disposed in the gap between generally parallel pole faces at the ends of the two magnet posts of an electromagnet. The resonant frequency of a YIG sphere is proportional to the strength of the magnetic field in which the sphere is placed, which is in turn dependent on the distance between the pole faces at the sphere location. Because of the anisotropic properties of YIG material, the resonant frequency is also dependent upon the angular position of the sphere about its axis. Each YIG sphere in a YIG tuned filter comprising more than one YIG sphere must have the same resonant frequency if the filter is to function properly. If the pole faces of the magnet are not parallel within about 2 nm, the spheres are subjected to sufficiently different field strengths that their resonant frequencies will be different if they have the same angular position. The resonant frequencies can then be made equal by rotating one or both spheres, but if the energizing current for the magnet coil is changed in order to change the magnetic field in the gap and hence the resonant frequency of the YIG spheres, the resonant frequencies of the two YIG spheres will no longer be the same.
One way to ensure that the resonant frequencies of the YIG spheres remain equal when the magnet coil's energizing current is changed is to grind the magnet posts so that the pole faces are parallel to within about 2 nm. However, this presents severe difficulties, especially when the electromagnet is assembled from separate pieces instead of being machined from a single block. When the electromagnet is assembled from separate pieces, the desired parallelism may be obtained by inserting a metal shim in the mounting structure for the pole pieces.